Switch responses to pure-ish faraday

[ksol]

This PR introduces breaking changes to the gem; readme and changelog will be updated subsequently.

In this PR, the Scalingo::API::Response is removed altogether. Instead, we use regular faraday objects, although with some minor additions:

1. boolean methods to quickly categorise the kind of error
2. exposes meta as a property on the response
3. exposes if the response is paginated, and the pagination info
4. exposes if the response if cursor-paginated, and the cursor info

On top of this, the unpack mechanism is now automated. If the response is successful and there is a single root key after dealing with the metadata, we automatically extract it. This

Each of those two functions is dealt with using faraday middlewares - both could be extract from this lib, but with so few code lines in each of them, the value is really low

[aurelien-reeves-scalingo]

Approved

Yet it could be nice to update the unreleased section of the changelog accordingly in the PR.